The present invention relates to a device for feeding sheets one by one from a stack of sheets to a transportation device for transporting the sheet to a process station, the device comprising a first low-pressure chamber with an integrated feeding table which supports the stack of sheets, a number of separately driven shafts which are positioned perpendicular to the direction of transportation and are arranged in the low-pressure chamber essentially equidistantly spaced from one another and which each carry a plurality of wheels with friction lining, which protrude through associated openings in the feeding table, and a sheet support which is arranged essentially vertically above the feeding table and at a distance from the feeding table which is somewhat larger than the thickness of a sheet. The invention also relates to a method for feeding sheets one by one from a stack of sheets to a transportation device for transporting the sheet to a process station.
The invention especially relates to, but is not limited to, a method and a device for feeding or punching of cardboard blanks, for instance corrugated cardboard, from a stack of blanks to a machine for applying text and/or symbols or for punching.
The problems which arise when feeding a (lowermost) sheet of a stack can be explained by the fact that, in practice, it is impossible to feed a sheet without a certain degree of skidding between feeding wheels and sheet, which causes insufficient repeatability. This is due to the fact that the friction between wheels and sheet changes with the continuously changing number of sheets in the sheaf, type of sheet (surface structure, thickness/weight etc.), changes in speed etc. In order to minimize the sliding between wheels and sheet, a large vacuum (negative pressure) has to be used. However, this implies that the next sheet is put down too fast and gets in contact with the retarding feeding wheels, which damages the sheets and wears the wheels out. There is also a risk that the next sheet is fed towards the front sheet support, which results in damage to the front edge of the sheet. This can also lead to the sheet feeding being interrupted when jamming occurs, i.e. two sheets (the one to be fed and the sheet on top thereof) are fed simultaneously into the gap between the sheet support and the feeding table and get stuck. Theoretically, this would be counteracted if a motor with a sufficient braking torque could be used. Then it would, theoretically, be possible to retard the wheel shafts in a considerably shorter time or over a considerably shorter distance. However, this is limited by the performance of commercially available motors which have either too high a maximum torque or too high a mass-moment of inertia. In order to counteract the above-mentioned problems, the vacuum has to be decreased, which has a detrimental effect on the repeatability when uncontrollable sliding (which also depends on the speed, the height of the sheaf etc.) appears.
A sheet-feeding device of the type defined above is already known from the U.S. Pat. No. 5,006,042. This known sheet-feeding device comprises a low-pressure chamber having an integrated feeding table on which a stack of sheets is intended to be placed, and a sheet support at a distance above the feeding table in the order of the thickness of one sheet. A number of shafts are arranged in the low-pressure chamber. The shafts carry a plurality of wheels which protrude through openings in the feeding table and serve to transport the lowermost sheet of the stack through the gap between the feeding table and the sheet support to a belt conveyor. Each shaft is driven by a separate motor. With reference to the reasoning above and to the fact that the distance is relatively large between the wheel shaft closest to the sheet support and on the one hand the sheet support and, on the other hand, the belt conveyor, there is an imminent risk that the sheets arrive inclined and/or with so-called index deviation at the belt conveyor with ensuing problems in the subsequent process station(s). No correction for the above-mentioned deficiencies is indicated in said patent. Furthermore, waiting sheets in the stack or sheaf, which due to frictional forces are pressed towards the sheet support (especially at a high level of vacuum), tend to get stuck with their front edge on the sheet support and, thus, be prevented from being correctly put down when sheets that are being fed have completed their feeding cycle. Often a corner of the front edge is pressed against the sheet support. Once the feeding cycle starts, the sheet is damaged or stuck on the sheet support and is not fed in a correct way.
Other problems that are related to sheet-feeding devices of the above-mentioned type are, for example, the following ones: If a xe2x80x9cnormalxe2x80x9d so-called cam (movement pattern) in the sheet-feeding cycle is used (see FIG. 7a), when changing the speed, the acceleration and retardation ramps (the inclination of the graphs) will change. This implies that, at decreased machine speed, lower retardation of the feeding wheels and longer time to stop the wheels are obtained, although a force for bringing about a faster stop is available in the motor. Consequently, there will be enough time for the next sheet of the sheaf to be sucked down onto the wheels before they have stopped. As a result, the surface layer of the sheet could be damaged by the wheels which spin intensively against the same (xe2x80x9crubbingxe2x80x9d) and the sheet is advanced to the front sheet support in an uncontrolled manner. Variations in parameters, such as size of sheet, height of sheaf, level of vacuum and machine speed, also result in a change in the total friction acting between sheet and wheels. The variations in friction give rise to variations in the sliding between sheet and wheels which always occurs in connection with the acceleration of a sheet. When the sliding varies, it appears as variations in the index of the sheet. Moreover, there is the ubiquitous stochastic variations in friction from one sheet to another depending on, inter alia, the individual surface structure of each sheet, turbulence in vacuum boxes (low-pressure chambers) etc. which give a stochastic index adding to the above-mentioned reasons for inadequate repeatability.
An object of the present invention is to provide a device and a method for feeding sheets which minimize risks of index errors and inclination of the fed sheets.
Another object of the invention is to provide a device and a method for feeding sheets which prevent a sheet from jamming on or below the sheet support.
Yet another object of the invention is to provide a device and a method for feeding sheets which reduce the risk of damage to the surface layer of the sheets.
Furthermore, an object of the invention is to provide a sheet-feeding device which can easily be adapted to stacks or sheaves of sheets of various dimensions.
These objects have been achieved by means of a sheet-feeding device as stated by way of introduction, which is characterized in that the device further comprises a second low-pressure chamber, between the first low-pressure chamber and said transportation device, having an integrated feeding table which forms an extension of the feeding table of the first low-pressure chamber, that a number of separately driven shafts are arranged in the second low-pressure chamber at essentially the same said distance from one another and having said distance between adjacent shafts in the first low-pressure chamber and in the second low-pressure chamber, respectively, each shaft in the second low-pressure chamber carrying a plurality of wheels with friction lining, which protrude through associated openings in the feeding table of the second low-pressure chamber, that at least one sensor is arranged between the second low-pressure chamber and said transportation device, the sensor being arranged to detect the position of the front edge of the fed sheet and to send signals to a control unit, and that the control unit is adapted to correct, if necessary, the position of the front edge of the sheet by controlling the drive motors of the shafts.
A method for feeding sheets by means of a sheet-feeding device as described above is characterized in that the wheels, from being immobile at the beginning of each feeding cycle, are caused to rotate by means of a control unit which is connected to the drive motors of the wheels and said process station, in order to accelerate the sheet, so that the it reaches its position reference value and its speed reference value depending on the working pace of the process station, and that the respective wheels, when the sheet leaves the wheel, are brought to a standstill by means of the maximum braking torque available.
Further developments of the device and the method according to the invention will be evident from the features that are stated in the subclaims.